When Luke Shaw lay prone on the Wembley turf after colliding with Dani Carvajal last Saturday, there was worry around the national stadium that the Manchester United defender was in for another long injury lay off.

Having started the season in good form for his club, Shaw was carried off on a stretcher, wearing an oxygen mask, early in the second half of England's 2-1 loss to Spain.

It was later revealed that, thankfully, Shaw was up walking and talking in the dressing room shortly afterwards.

England medics ran test and soon found that he had suffered concussion; he remained under close watch that evening, before being released by the Three Lions to return to United's care on Monday.

Since then, Shaw hasn't been involved in full training, with the club's doctors and medical staff keeping tabs on his recovery.

There was widespread worry after Shaw's collision with Carvajal (Image: Getty Images Europe)

The England defender was later found to have suffered a concussion (Image: PA)

FA guidelines state a player who suffers concussion should not play for 14 days.

It declares: "The graduated return to play (GRTP) protocol should be followed in all cases.

"This staged programme commences at midnight on the day of injury and stage 1 (initial rest period) is 14 days in all players unless they are in an enhanced care setting.

"In all cases, progression to stage 2 of the GRTP can only occur if the player has no symptoms"

That would rule Shaw out of this weekend's trip to Watford , plus the Champions League clash against Young Boys and next weekend's meeting with Wolves at Old Trafford.

“I don’t know [if he will play] - we still have a training session,” said Mourinho at his press conference.

“Contrary to some news, to some opinions, by the protocol point of view and according to our doctor, he will be free to play.

“The only situation we have to analyse is if you are going to play him when, during the week, he was not training with the team or if, even, he decides to play.”

The reason Shaw is able to be considered is down to United's state-of-the-art medical facilities being considered as an "enhanced care setting".

(Image: TheFA.com)

A player - over the age of 19 - can return to action fully after six days in this scenario. They must still go through the same six stages; again, in an "enhanced care setting" stages 2-5 each comprise a 24 hour period.

But Stage 1, the initial rest period, is only 24 hours also; so long as symptons don't reoccur, a player can move onto stage 2 straight away. Stages 2-5 then comprise four days - provided the player remains symptom free, allowing the player, in this case Shaw, to return to action on the sixth day.

According to FA guidelines on players' returning to action and 'enhanced care settings', there are seven criteria that must be fulfilled for a player to return to action in a shorter time frame; if any element is absent, then a player must wait the full 14 days.

The minimum criteria for an Enhanced Care Setting are as follows:

There is a doctor with training and experience in the management of concussion/traumatic brain injury in sport available to closely supervise the player’s care and GRTP, and clear the player prior to RTP.

There is a structured concussion management programme in place for the player as outlined below:

Baseline SCAT3 and/or computerised neuro-psychometric/cognitive testing of the player has been conducted prior to the injury

Clinical serial multimodal assessment of the player occurs post-concussion to guide the recovery protocol

Acknowledging that more than one area of brain function can be affected by concussion, this will involve formal documented assessment of areas such as cognitive function, emotional wellbeing, neurological function and any physical trauma sustained

A formalised GRTP programme with regular SCAT3 or equivalent assessments is followed and recorded in the player’s medical records

The player has access to a multi-disciplinary team including neuropsychology / neurology / neurosurgery specialists and other clinicians as required to supervise the return to play and instigate any treatment or investigation required should the RTP progression not be straightforward

A formal and documented concussion education programme exists for coaches and players in the club or team involved

With all of those protocols followed, United will therefore be allowed to use Shaw against the Hornets - provided he comes through Friday's training session unscathed, that United's medical staff are fully convinced of his recovery and proves his fitness.

Ashley Young, England's left wing-back at the World Cup but on the outside looking in for United so far this term, is ready to deputise.